Which type of non-functional requirement ensures that an updated database can return back in the event of update failures?

The correct answer focuses on reliability, which is a crucial non-functional requirement in system design. Reliability refers to the ability of a system to consistently perform its intended functions without failure over time. In the context of databases, it ensures that the system can recover accurately from failures, including update or transaction failures. A reliable system is designed with mechanisms such as transaction logging, backup, and restore procedures to maintain data integrity and availability, even in the event of an update failure.

When a database faces an update failure, reliability ensures that it can revert to a stable state, preserving previous data and providing a seamless recovery process. This characteristic is especially important for applications that handle critical data where consistency and accuracy must be maintained at all times.

The other options represent different aspects of a system’s performance. Availability pertains to the system's uptime and accessibility to users, maintainability focuses on how easy it is to update and manage the system, and scalability involves the system's capacity to handle increased load or expand to accommodate growth. While these features are essential for a robust system, they do not specifically address the need for data recovery and integrity in the event of update failures, which is why reliability is the key focus here.